2,3-butanediol is an alcohol (represented by CH3CHOHCHOHCH3) having four carbons and two hydroxyl (—OH) groups and can be chemically and catalytically converted into 1,3-butadiene, which is a raw material for preparation of synthetic rubbers, and methyl ethyl ketone (MEK), which is a fuel additive and a solvent (Ji et al., Biotechnol. Adv., 29: 351, 2011). 2,3-butanediol is a very important industrial intermediate since 2,3-butanediol can be used as an octane booster through mixing with gasoline (Celinska et al., Biotechnol. Adv., 27: 715, 2009).
2,3-butanediol can be produced by chemical synthesis and microbial fermentation. However, due to high production costs, 2,3-butanediol has not been produced on a commercially viable scale. In recent years, with the rapid development of techniques for producing 2,3-butanediol through microbial fermentation, price surge for fossil fuel derivatives, and tightened international regulations on environmental contamination, there has been a growing focus on the importance of finding bio-based methods for producing 2,3-butanediol through microbial fermentation.
Research into bio-based methods for producing 2,3-butanediol through microbial fermentation has been performed in divided categories such as optimization of fermentation processes (temperature, pH, dissolved oxygen, and the like) and searching for microorganisms (identification of microorganisms, characterization of physiological properties of identified microorganisms, performing mutation and genetic engineering, and the like). In view of optimization of fermentation processes, various conditions for ensuring effective production of 2,3-butanediol, such as temperature, pH, dissolved oxygen concentration, and the like have been identified (Ji et al., Bioresour. Technol., 100: 3410, 2009; Nakashimada et al., J. Biosci. Bioeng., 90: 661, 2000; Nakashimada et al., Biotechnol. Lett., 20: 1133, 1998). However, production of 2,3-butanediol through microbial fermentation under these conditions still has problems of low productivity and yield, which makes direct application thereof to commercial processes difficult. Furthermore, such fermentation has a disadvantage in that various byproducts such as organic acids including lactic acid, alcohols including ethanol, and the like are produced together with 2,3-butanediol during fermentation.
Production of byproducts not only lowers yield of 2,3-butanediol as compared with raw biomaterials, but also requires great cost for separation and purification of 2,3-butanediol upon harvesting 2,3-butanediol from the culture solution. Accordingly, the development of microorganisms related to production of 2,3-butanediol has been performed in a direction of decreasing byproducts. Ji et al., have succeeded in partially suppressing production of organic acids as byproducts by exposing a wild type Klebsiella oxytoca to ultraviolet radiation (UV) as a physical/chemical mutation method (Ji et al., Biotechnol. Lett., 30: 731, 2008). Further, it was possible to enhance production of 2,3-butanediol by applying ion beams to Klebsiella pneumonia, thereby increasing biomass consumption rate (Ma et al., Appl. Microbiol. Biotechnol., 82: 49, 2009). In research relating to byproduct reduction through selective genetic engineering, mutant microorganisms made by deleting a gene (ldhA) responsible for production of lactic acid as one of major byproducts exhibited the best performance under general conditions. In addition, there have been examples that genes (adhE, aldA) responsible for production of ethanol are deleted in order to decrease production of ethanol as a byproduct (Ji et al., Appl. Microbiol. Biotechnol., 85: 1751, 2010). In some examples, the activity of pyruvate formate lyase responsible for generating formic acid in lactic acid bacteria (LAB) is decreased (WO2010/037114 A1).
The present inventors have identified a gene having conversion activity on 2,3-butanediol, and have also identified that recombinant microorganisms in which the activity of the gene is suppressed show an enhanced ability to produce 2,3-butanediol and consumption of the produced 2,3-butanediol is prohibited. Based on this finding, the present invention has been completed.